The present invention relates generally to overvoltage surge arresters and particularly, but not exclusively, to such arresters for high voltage direct current applications.
Arresters protect the insulation of electrical power systems by momentarily reducing the impedance to ground of that portion of the system carrying an overvoltage surge, so that the undesired variation in available energy from the system due to the surge is safely drained off. Thus, a high voltage arrester may be regarded as a high speed, voltage sensitive, high current switch. The switching function in conventional arresters has been performed by a series combination of valve units and electrode gap units. The valve units are disc-shaped blocks of non-linear resistance, or varistor material which exhibits a decreasing resistance with increasing voltage. The gap units include a pair of horn-shaped gaps inside a special sparking chamber designed to aid in extinguishing an arc between the electrodes. There may be other components provided for the gap unit to control the establishing and extinguishing of the arc. For higher voltage arresters a plurality of valve units and gap units are interspersed in a series stack inside an insulating housing, usually a porcelain tube with metal end caps.
When such a conventional arrester is subjected to an overvoltage, one or more gaps spark over. This completes the circuit through the valves to ground. Current passes through the arrester to ground until the normal system voltage is once again established. At normal system voltage, the increased resistance of the valve section decreases the arrester current to a value insufficient to sustain arcing in the gap units. Thus the arcs are extinguished, and the arrester is once again an open switch.
The valve block resistance is determined by the relationship I=KV.sup.n, where I represents the current, K represents a constant, V represents the voltage across the block, and n represents a numerical value which for conventional silicon carbide blocks is about 4 and which is referred to by those in the art of surge arresters as the "exponent" to describe the degree of non-linearity of a particular valve block varistor material. There are "low exponent" materials with an exponent of less than about ten and "high exponent" materials with an exponent greater than about ten.
It is recognized that the use of high exponent material such as a zinc oxide varistor compound for the valve blocks will result in a very substantial reduction in the magnitude of the follow current, without raising the arrester voltage above the desired limits during a discharge. The follow current can be reduced sufficiently that no additional current limiting function is required prior to simple clearing of the current by a simple series gap section, a section of simple gap units which are not provided with coils or other such features for limiting the follow current to a magnitude which will permit clearing. An arrester with a high exponent valve section and a simple series gap section can have the advantages of reduced cost, reduced size, and improved performance for both long voltage surges, such as switching surges, and isolated short surges, such as lightning surges. It is found, however, that when such an arrester is subjected to two or more lightning surges in quick succession, such as would result from a multiple lightning stroke, the performance of the arrester is severely degraded after the clearing of the first surge. The degraded performance is characterized by an altered sparkover voltage level for the arrester which permits the arrester voltage to exceed the desired sparkover voltage before the discharging process is initiated. Consequently, the arrester will fail to protect the system against such multiple surges.